examples/muxing: fix av_frame_free() not called when got_packet is false
examples/muxing: fix memleaks in resampler

  - do not allocate resample dst buffer when resample is off
  - free sample buffers in addition to freeing data pointer arrays
examples/muxing: use S16 sample_fmt for resample src regardless of codec sample_fmt

We generate S16 samples and we should allocate the right buffer